View Full Version : Trying to replace a 24 vac transformer
Dec 4, 2004, 11:27 AM
I need instructions on how to wire the junction box where the 120 vac power, the fan limit, and the blower motor wires all come together. I undid all the wires and got confused on how to put it back together.
Here are the facts: 25 year old Rheem gas hot air upright furnace.
Three wires from the blower motor. Black, red, and white.
Two wires from the fan limit control. Both black (one with writing).
Three wires from the transformer for 24vac. Red, black, nad yellow.
Two wires (LINE) for the 120vac to the transformer. (I am sure about that part)
I cannot figure out how to hook up the fan limit and the blower motor. Please help. Thanks.
Dec 4, 2004, 02:11 PM
This is a gray area I am short the experience I need to give you a good answer. I am not familiar with your particular furnace, and they can be wired up different ways. It sounds like you have some options you don't need for your system, always confusing. You may not be able to work this out without an ohm or volt meter. The limit switch is likely an normally open switch that closes when the air is warmed up after the gas comes on. Is there a relay that controls the blower?
Start with the transformer. The black and white line wires may be connected to either of the primary leads. Check the resistance between pairs of the red, black, and yellow. Two pairs should an equal resistance and the third pair the sum of the other 2. With the power on, they should give 24 volts and be the ones you want. If one of them is the red one, connect it to the red wire that goes to the thermostat. Connect the other one of that pair to a likely blue wire that goes to the gas valve, the fan relay, and anything such as an air conditioner. The third wire is not used, being for rectifying to DC in other applications. Connect the white wire from the blower motor to the white line wire along with the one transformer. There should be a smaller white wire that comes from the thermostat and connects to the gas valve plus another loose wire. Connect that wire to one of the leads of the limit switch. Connect the other lead from it to the other terminal of the blower relay coil. The red and black wires may give a option of various speeds for the blower. You may need to connect one, the other, or both to one of the switch terminals on the relay. Unless you find a wiring diagram, you may just have to study the wires, what you remember, and if nothing else, go by what sounds like the blower should run. Sometime there will be a wiring diagram on the furnace, or under a cover. The black line wire connects to the other switched terminal. When the thermostat signals for heat, power should flow to the red wire to the thermostat and then back the white wire to the gas valve and the blower limit switch. The gas will come on. When the limit switch closes, the blower will start. It is possible that there is no relay for the blower. In that case connect the black line wire to one lead to the limit switch, and which ever wires from the blower motor you think you need.
It is possible the transformer is wired to provide 2 separate 24 volt sources with one wire common to both. If that is the case, one pair will go to the thermostat and gas valve, the other to the blower relay and limit switch. That way, the blower would continue running until the air in the furnace cools off.
Check things out. If my directions don't seem to match your furnace, get back with me telling me what you find. Somebody else here may be able to help too.
Dec 4, 2004, 04:01 PM
I am hooking up heat only, but the furnace can do a/c.
Dec 4, 2004, 07:34 PM
Start with the transformer. The black and white line wires may be connected to either of the primary leads. Check the resistance between pairs of the red, black, and yellow. Two pairs should have an equal resistance and the third pair the sum of the other 2. With the power on, they should give 24 volts and be the ones you want. One needs to go directly to the gas valve. The other must somehow go to the thermostat and back to the gas valve when it calls for heat. There may be another another wire in the furnace that connects to the likely red wire to the thermostat.
It is possible that there is no relay for the blower. In that case connect the black line wire to one lead to the limit switch, and which ever wires from the blower motor you think you need to the other. Then the blower should run from the time the furnace gets hot enough after the gas comes on to close the switch, until it cools off after the gas shuts off.
Connect the white wire from the blower motor to the white line wire along with the one transformer lead.